128 Outlines of Horticultural Chemistry. 



remains unaltered. That the first ever occurs in practice, 

 arises from the faulty cultivation of the soil ; for, if properly- 

 drained, however retentive it may be, no natural deposition 

 of moisture is ever too abundant or continuous. Some seeds, 

 as those of aquatics, succeed only when completely immersed in 

 water ; others, as those of the lemon, will often germinate with 

 the unassisted moisture of their own pericarp. 



All seeds require a certain degree of heat ; none will ger- 

 minate at a temperature so low as that of freezing water, yet 

 the greatest degree of cold is not injurious to them, if germi- 

 nation has not commenced. Every seed appears to vary in the 

 degree of heat which it requires before vegetation commences, 

 though an increase above such temperature, if not excessive, 

 always accelerates the progress. Adanson found that seeds 

 which naturally do not germinate in a less space of time than 

 twelve hours, may, by an increase of heat, be made to do so 

 in three hours. Seeds ripened in high latitudes, or at great 

 heights, and consequently in a climate whose average temper- 

 ature is much lower than that of countries nearer the equator 

 or of less elevation above the sea, germinate much more 

 quickly when sown in these latter climates, than if re-sown 

 where they were produced ; a fact which defies explanation, if 

 plants are devoid of sensation. 



The experiments of Ingenhouse and Sennebier evince that 

 light retards germination ; and some which were tried under 

 my own inspection afforded confirmatory results. This fact 

 has long been practically acknowledged, by the cultivators of 

 the soil burying their seed beneath its surface. 



These facts hold out some beacons worthy of being at- 

 tended to, as guides for the operation of sowing. They point 

 out that every kind of seed has a particular depth below the 

 surface, at which it germinates most vigorously, as securing to 

 it the most appropriate degree of moisture, of oxygen gas, and 

 of warmth. From a quarter of an inch to two inches beneath 

 the surface, appears to be the limits for the seeds of plants 

 usually the objects of cultivation ; these, however, must vary 

 for the same seeds in different grounds and countries. It must 

 be the least in aluminous soils, and dry climates. Sowing 

 should in general be performed in dry weather, especially on 

 heavy soils, not only because of the greater saving of labour, 

 but because it prevents the seed being enveloped with a coat 

 of earth impermeable by the air, " which," says Sir H.Davy, 

 " is one cause of the unproductiveness of cold, clayey soils." 

 Perhaps the time at which any ground may be raked with the 

 greatest facility, is as good a practical criterion as any, to judge 

 when it is most fit for sowing. In general, if clay does not 



